The invention relates to a switching device which is more particularly described as a device wherein metallic contacts are used as switching elements.
The opening and closing of contacts are the primary objectives of switching devices utilizing metallic contacts as switching elements. Thus, it is important in many applications of such devices that the contacts themselves are made reliable and long lasting. Relays used in telephone switching circuits, distributor points used with internal combustion engines, and starting relays used with various household appliances are several of the many possible applications requiring long lasting, reliable contacts. For instance, contacts of switching devices used in telephone switching circuits are expected to provide reliable service for 40 years or more. A long lasting contact has to resist erosion caused by electrical arcing in order to maintain a reasonably low electrical resistance.
Studies have shown that erosion of contacts on closure and separation is caused almost entirely by electrical arcs occurring between two mating contact surfaces when there is potential difference between the two surfaces. When there is no arcing between two mating contact surfaces, there is no erosion, and contact life is then generally limited by mechanical wear of the contact surfaces.
In a switching device, such as a wire spring relay shown in U.S. Pat. No. 2,562,091 issued to H. C. Harrison, arcing erosion of mating contact surfaces is a problem in that it increases the contact resistance and reduces the useful life of the contacts.
Therefore it is an object to provide a switching device having reliable, extended life contacts.
Another object is to improve the operation of a switching device by reducing the effect of erosion caused by arcing.
These and other objects are realized in an illustrative embodiment of the invention in which a switching device has a fixed contact and first and second electrically connected, movable contacts. The movable contacts are moved so that the first movable contact closes with the fixed contact before the second movable contact closes to the fixed contact and so that the first movable contact opens from the fixed contact after the second movable contact opens therefrom.
One embodiment is arranged for moving the two movable contacts so that the first movable contact closes with the fixed contact before the second movable contact closes to the fixed contact and so that the first movable contact opens from the fixed contact after the second movable contact opens therefrom.
In another embodiment, the first contact is made of a material which resists erosion caused by electrical arcs.